1. Field of the Invention
The present invention relates generally to temperature measuring apparatus of the type employing a temperature responsive electrical element, such as a thermistor, as the temperature sensor. Specifically, the invention relates to that form of such apparatus which provides both analog and digital outputs which are representative of the temperature sensed by the sensor.
2. Description of the Prior Art
It is well known to provide temperature measuring apparatus of the form noted above which produces both analog and digital outputs which represent the value of the temperature to which the sensor of the apparatus is exposed. An example of such known apparatus is that which is disclosed in FIG. 11 on Page 2-51 of the book entitled "Process Instruments And Controls Handbook", Second Edition, edited by Douglas M. Considine, and published by the MacGraw-Hill Book Company in 1974. In such apparatus, the output of the sensor is amplified by an analog amplifier, and the output voltage of the amplifier is the analog output which is representative of the sensed temperature. The digital output of the apparatus is obtained by applying the analog output voltage to an analog to digital converter, such as a voltage to count converter. The output of the latter is the digital output of the apparatus.
In apparatus as just described, compensation for non-linearity in the resistance vs. temperature characteristic of the sensor is usually done by suitably adjusting the gain of the analog amplifier. Adjustments for different scale factors are usually effected in a second analog amplifier, since making such adjustments to the first or compensation amplifier necessitates making similar adjustments to the compensation. A disadvantage inherent in such apparatus is that whatever errors appear in the analog output signal, due to the above-noted amplifiers, similar degrade the digital output.